1. Field of the Invention
The present invention relates to the monitoring of signals applied by a central office to a subscriber line and more paticularly to a ring sensing circuit for use by a service evaluation system to detect low frequency AC ringing signals applied to a subscriber line by a switching center. The present circuit designed for inclusion in such a service evaluation system provides a digital indication of the application of ringing current to a subscriber line to an associated microprocessor controller.
2. Description of the Prior Art
In a telephone system it is necessary that an evaluation of traffic flow (telephone calls) be made in order to determine the quality and quantity of equipment that must be provided in a switching center. This equipment provides various supervisory signals such as dial tone and ringing to the subscriber line. For example, ringing is accomplished via the application of a low frequency AC voltage superimposed upon a DC bias voltage of either polarity applied at the switching center to the tip and ring leads of a line circuit.
Existing circuitry typically senses the application of such ringing voltages and supplies an analog signal as its output. A disadvantage of this type of circuit is that to obtain a digitized status of the ringing condition an analog to digital converter must be utilized. Such converters are usually expensive.
One solution is taught by U.S. Pat. No. 3,842,216 issued to James Owen et al on Oct. 15, 1974, which teaches the use of a neon lamp and a phototransistor. The output of the phototransistor is applied to a digital frequency filter for conversion. Components such as a neon lamp, phototransistor and a digital filter network are typically very expensive. Thereby, such circuitry is rendered prohibitive in applications where large numbers of such sensing circuits are required.
U.S. Pat. No. 3,808,377 which issued to applicant on Apr. 30, 1974, teaches the use of a sensing circuit including a pair of voltage divider networks with a capacitor connected between their mid-point and acting as a voltage differential detector measuring the voltage differential between the mid-points of the respective dividers. This type of circuit is useful, however, only for the detection of subscriber off-hook condition.
Therefore, it is an objective of the present invention to provide an inexpensive line ringing sensing circuit for use by a microprocessor controlled service evaluation system to provide a digital indication of the application of ringing current to a subscriber line. In addition, it is a further objective of the present invention to provide a simple circuit so that several of such circuits may be placed on a single integrated circuit using thick film techniques or incorporated on the same integrated circuit unit with the above noted off-hook detector.